The present invention relates to a method of positioning thin flat objects in a processing machine.
Such machines are used particularly in the printing and packaging industry, for example for making cartons from thin flat objects such as preprinted sheets. These sheets are taken from a stack situated upstream of the machine and passed into an insertion station by an inserter so that they can be positioned in clamping bars connected at regular intervals to a subsequent endless chain drive. The latter takes the sheets through the various later processing stations of the machine. Such stations typically cut the sheets, eject the cut waste, and collect these sheets in a stack.
In discontinuous transport, the chain drive moves and stops periodically in such a way that, during each movement, all the clamping bars gripping a sheet are moved from one station to the next station downstream. If good-quality printing or production is desired, the positioning of the sheets within the various successive stations is an operation of the utmost importance. If a printed sheet is to be cut, it will be realized that the sheet has to be positioned with great accuracy in the cutting station: care must be taken to ensure that the cutting tools, for example the cutting blank of a flat bed die press, is in perfect register with the image previously printed on the sheet.
Patent CH 690 470 describes a device for ensuring the quality of production of a press for producing packaging. For this purpose the device comprises a video camera designed to read not only the print-related reference marks but also a mark designed as a reference for the cutting position. These reference marks are placed on the front waste of the sheet which is held by the clamping bar. The cutting mark is made by a perforator connected to the cutting tools. This perforator makes a hole in the front waste of the sheet at the same time as the sheet is being cut. Further downstream another device marks sheets identified by the camera as defective, namely sheets with an out-of-tolerance divergence between the printed image and the cutting.
In patent EP 448 943, reference is made to clamping bars connected at their ends to the chains of the conveyor by so-called “floating” attachment systems. These attachment systems allow each clamping bar to be stopped and immobilized momentarily in a position rigorously determined in each station by means of a mechanical clamping system and by means of the elastic floating between the clamping bar and the chain drive. By means of such a system the thin flat object can be positioned with great accuracy, both at the moment of its insertion into the clamping bar and at each of the processing stations in which this system can be used to keep the bar in perfect register with the tools of this station.
However, such a system has the disadvantage, either of not being applicable to all machines, or of not being the most appropriate system which could be fitted to certain types of machines, notably machines handling corrugated board.
Patent EP 1 044 908 relates to a device and method for positioning thin flat objects in an insertion station. From a shelf situated in a rear starting position, this method consists in engaging means for fixing a thin flat object on the shelf and then causing actuators to move it forwards on the basis of the position of the thin flat object on the shelf. As a result, the front edge of the thin flat object is brought towards, stopped and then released in a predetermined position in the clamps of the clamping bar of the conveying device before the shelf has been finally returned to the starting position. To allow the shelf to be moved an appropriate distance forwards, if necessary sideways or obliquely, optoelectronic means read the coordinates of the position of the thin flat object and work out the necessary movement to position it correctly in the clamping bar.
Although optimized to position the thin flat object in the clamping bar on the basis of its initial starting position, very visible errors have nonetheless been found between the printed image and the cutting performed on these objects in machines fitted with such devices. These errors, both lateral and longitudinal, persisted in spite of the fact that the positions of the thin flat objects had been calculated correctly from the print-related reference marks, which themselves had been read properly by the optoelectronic means.